1. Field of the Invention
The present invention relates to a data transfer circuit, a solid-state imaging device typified by a complementary metal oxide semiconductor (CMOS) image sensor, and a camera system, and in particular, to a solid-state imaging device including a column analog-to-digital converter, and a camera system.
2. Description of the Related Art
A CMOS image sensor including a column-parallel analog-to-digital converter (ADC) has been proposed (see, for example, W. Yang, et Al., “An Integrated 800×600 CMOS Image System”, ISSCC Digest of Technical Papers, pp. 304-305, February, 1999).
FIG. 1 is a block diagram showing an example of the configuration of a solid-state imaging device 1 (CMOS image sensor) including a column-parallel ADC.
The solid-state imaging device 1 includes a pixel array section 2 serving as an image pickup section, a row scanning circuit 3, a column scanning circuit 4, a timing control circuit 5, an ADC group 6, a digital-to-analog converter (DAC) 7, a counter 8, and a subtracting circuit 9.
The pixel array section 2 is formed by unit pixels 21 arranged in a matrix, each unit pixel including a photodiode and an in-pixel amplifier.
In the solid-state imaging device 1, the timing control circuit 5, the row scanning circuit 3, and the column scanning circuit 4 are used as control circuits for sequentially reading signals from the pixel array section 2. The timing control circuit 5 generates internal clocks. The row scanning circuit 3 controls row addresses and row scanning. The column scanning circuit 4 controls column addresses and column scanning.
The ADC group 6 is formed by a plurality of ADCs arranged. The ADCs include comparators 61 and memory devices 62. The comparators 61 compare a ramp waveform RAMP formed by changing, in steps, a reference voltage generated by the DAC 7, and analog signals obtained from the unit pixels 21 for each of row lines H0, H1, . . . via column lines V0, V1, . . . , and the memory devices 21 store count results of the counter 8, which counts comparison times.
Each ADC of the ADC group 6 has an n-bit digital signal conversion function and is disposed for each of the column lines V0, V1, . . . to form a column-parallel ADC block 63.
Outputs of the memory devices 62 are connected to horizontal transfer lines 64 having a 2n-bit width.
In addition, 2n sense circuits corresponding to the horizontal transfer lines 64, the subtracting circuit 9, and an output circuit are disposed.
Here, an operation of the solid-state imaging device 1 (CMOS image sensor) is described, with the operation associated with the timing chart shown in FIG. 2 and the block diagram shown in FIG. 1.
After first-time reading from the unit pixels 21 in any row Hx to the column lines V0, V1, . . . becomes stable, a step-like ramp waveform RAMP in which a reference voltage is time-changed is input from the DAC 7 to one comparator 61, and the comparator 61 compares the input waveform with a voltage in any column line Vx.
In parallel to input of the step-like ramp waveform RAMP, the counter 8 performs the first counting.
Here, when the ramp waveform RAMP is equal to the voltage in any column line Vx in voltage, the output of the comparator 61 is inverted, and, at the same time, a count in accordance with a comparison period is stored in a corresponding memory device 21. In the first-time reading, reset components ΔV in the unit pixels 21 are read. The reset components ΔV include, as offsets, noise varying for each unit pixel 21.
However, in general, a variation in the reset components ΔV is small, and a reset level is common to all the pixels. Thus, an output from any column line Vx is approximately known.
Therefore, at the time of the first-time reading of reset components ΔV, by adjusting the ramp waveform (RAMP) voltage, the comparison period can be reduced. In this example, in a count period (128 clocks) for seven bits, ΔV comparison is performed.
At the time of the second-time reading, in addition to the reset components ΔV, signal components in accordance with the amount of light incident in each unit pixel 21 are read and an operation similar to that performed in the first-time reading is performed.
In other words, after second-time reading from the unit pixels in any row Hx to the column lines V0, V1, . . . becomes stable, the step-like ramp waveform RAMP in which a reference voltage is time-changed is input from the DAC 7 to the comparator 61, and the comparator 61 compares the input waveform with a voltage in any column line Vx.
In parallel to input of the step-like ramp waveform RAMP, the counter 8 performs the second counting.
Here, when the ramp waveform RAMP is equal to the voltage in any column line Vx in voltage, the output of the comparator 61 is inverted, and, at the same time, a count in accordance with the comparison period is stored in the corresponding memory device 62.
At this time, the first and second counts are stored at different locations in the memory device 62.
After the AD conversion period finishes, n-bit digital signals (in the first-time and second-time reading) stored in the memory device 62 by the column scanning circuit 4 pass through the 2n horizontal transfer lines 64. The sense circuits and the subtracting circuit 9 perform processing so that (the signal in the second time)−(the signal in the second time), and output the processed result to the exterior. After that, a similar operation is sequentially repeated for each row, whereby a two-dimensional image is generated.